Conventional MOS technology employs transistor gates including a metal over a doped polysilicon that is, in turn, over a gate dielectric. With the scaling of transistors to smaller sizes, the gate dielectric has transitioned from conventional silicon dioxide to higher-dielectric constant materials, such as hafnium dioxide (HfO2) and hafnium silicon oxide (HfSiO), to achieve higher oxide capacitance.
The conventional polysilicon gate that was used with silicon dioxide is difficult to use with Hf-based dielectrics due to the interaction at the interface of the gate and gate dielectric and to fermi-level pinning. To overcome these problems, a metal gate, such as a transition metal nitride, is often used. However, a major drawback of using metal gates is the inability to tailor the work function of the metal gates. Consequently, it is difficult to achieve a low threshold voltage with metal gates.
One prior solution was to dope the metal gate with arsenic. Arsenic is a donor atom used for doping silicon. Accordingly, in an nMOS device, for example, an arsenic implant can result in channel counterdoping, which results in the degradation of the transistor's performance.
It is therefore desirable to achieve a metal-gated nMOS transistor having a lower threshold voltage without such disadvantages.